The present invention, in some embodiments thereof, relates to an actuator and, more particularly, but not exclusively, to a piezoelectric-ferroelectric actuator device operating, at least in part, in the nonlinear ferroelectric range.
An actuator is a mechanical device for moving or controlling another device. The actuator generally provides force or motion in response to an applied stimulus. Many commonly used actuators are inductive in nature. However, from a purely theoretical point of view, piezoelectric actuators, which are capacitive in nature, exhibit much more desirable mechanical and electrical characteristics. Piezoelectric materials are characterized by an ability to deform mechanically in response to an applied electric field, as a result of the inverse piezoelectric effect. Piezoelectric actuators have a very efficient coupling of energy from electrical charge to mechanical strain, which results in a high bandwidth, large force output, fast actuation stroke and low resistive heating. The actuator stiffness is determined by the modulus of the material used for the actuator, rather than by an inherently weak magnetic coupling.
Piezoelectric stack actuators are formed of several layers of piezoelectric material. An electric field is applied to these actuators in the thickness direction, and the resulting force and displacement are also in the thickness direction. Actuators of this type provide sufficient force at the expense of displacement, and are suitable for many applications such as those required micro- and nano-positioning, as well as for many other mechanical and structural applications. Heretofore, piezoelectrics have been employed for minor control, ink jet nozzles, ultrasonic medical devices, high frequency audio speakers, miniature valves and the like.
During electrical excitation in the linear piezoelectric range, the amount of displacement which a piezoelectric material undergoes is relatively small. Piezoelectric actuators are therefore characterized by short travel range (also known in the literature as “stroke”).
Several studies have been directed to investigate the ferroelectric properties of some piezoelectric materials such as PZT and PMN [Mitrovic et al., (2001), “Response of piezoelectric stack actuators under combined electro-mechanical loading”, International Journal of Solids and Structures 38(24-25):4357-4374; Chaplya et al., (2001), “Dielectric and piezoelectric response of lead zirconate-lead titanate at high electric and mechanical loads in terms of non-180 degrees domain wall motion” Journal of Applied Physics 90 (10): 5278-5286; Shilo, et al., (2007) “A model for large electrostrictive actuation in ferroelectric single crystals” International Journal of Solids and Structures, 44(6):2053-2065; Burcsu et al., (2004) “Large electrostrictive actuation of barium titanate single crystals”, Journal of the Mechanics and Physics of Solids 52(4):823-846].
The ferroelectric range of a piezoelectric-ferroelectric material is characterized by self heating, material parameter degradation, fatigue after millions of cycles and complex relations between the strain, electric displacement, stress and electrical field. These complex elations are nonlinear, non-monotonic and hysteretic. Additionally, in the ferroelectric range, there is an abrupt directional change of the spontaneous polarization and strain of the unit cells (the so called “domain switching phenomenon”). As a result, the strains and the electrical displacement, as well as the elastic, dielectric, and piezoelectric tensors, become strongly dependent on the domain state of the grain and they may discontinuously vary through the mechanical and/or electrical loading process. Thus, although relatively large strains can evolve within the ferroelectric range, traditional actuation devices do not operate in this range since the nonlinear behavior and discontinuous variations are hard to be controlled.
Additional background art includes Balke et al., (1997) “Fatigue of Lead Zirconate Titanate Ceramics I: Sesquipolar Loading” Journal of the American Ceramic Society, 90(4):1088-1093; Chaplya et al., (2006), “Durability properties of piezoelectric stack actuators under combined electromechanical loading”, Journal of Applied Physics 100(12):124111-1:124111-13; and Lesieutre et al.